In one known self propelled vibrating road roller the compactor drum is vibrated at 1700 rpm by a dual amplitude vibratory mechanism which utilizes a swinging weight. The compactor drum rotates at vehicle speed as the vehicle moves forward. The drum is supported on the vehicle frame by anti-vibration mountings. An axial, reversible drive shaft at the center of the drum is driven by a hydraulic motor from the vehicle. The drum shaft has, in addition to a fixed eccentric weight, a hinge pin parallel to the axis of the shaft, a pocket for accommodating the swinging weight and stops to limit the travel of the swinging weight. When the shaft rotates in one direction, the swinging weight turns on the hinge pin to an eccentric position which adds to the fixed eccentric weight and increase the eccentric moment. When the shaft rotation is reversed by the operator, the hinged weight turns again on the hinge pin to the extent limited by the remaining stop and subtracts from the eccentric moment. The impact of the weight on the stops at each halt, start and reversal eventually produces metal particles in the mechanism which contaminate the bearings causing consequential failure and the drum must be disassembled to give access for repair.